In order to understand the nature of human embryos I first distinguish between active and passive potentiality, and then argue that the former is found in human gametes and embryos (even in embryos in vitro that may fail to be implanted) because they all have an indwelling power or capacity to initiate certain changes. Implantation provides necessary conditions for the actualization of that prior, active potentiality. This does not imply that embryos are potential persons that do not deserve the same respect as actual persons. To claim that embryos become persons is to understand the predicate person as a phase sortal, roughly equivalent to adult person. This entails that we would not be essentially persons. In order to explain the traditional understanding of person as a proper sortal rather than a phase sortal, the author distinguishes between proximate and remote potentiality, and shows that, unlike feline embryos, human embryos, by their genetic constitution, possess the remote potentiality to later exercise the typically human activities. It follows that they are already persons essentially.